The Effect of Natural LCAT Mutations on the Biogenesis of HDL

Novel therapeutic opportunities for familial lecithin:cholesterol acyltransferase deficiency: promises and challenges

PURPOSE OF REVIEW

Genetic lecithin:cholesterol acyltransferase (LCAT) deficiency is a rare, inherited, recessive disease, which manifests as two different syndromes: Familial LCAT deficiency (FLD) and Fish-eye disease (FED), characterized by low HDL-C and corneal opacity. FLD patients also develop anaemia and renal disease. There is currently no therapy for FLD, but novel therapeutics are at different stages of development. Here, we summarize the most recent advances and the opportunities for and barriers to the further development of such therapies.

RECENT FINDINGS

Recent publications highlight the heterogeneous phenotype of FLD and the uncertainty over the natural history of disease and the factors contributing to disease progression. Therapies that restore LCAT function (protein and gene replacement therapies and LCAT activators) showed promising effects on markers of LCAT activity. Although they do not restore LCAT function, HDL mimetics may slow renal disease progression.

SUMMARY

The further development of novel therapeutics requires the identification of efficacy endpoints, which include quantitative biomarkers of disease progression. Because of the heterogeneity of renal disease progression among FLD individuals, future treatments for FLD will have to be tailored based on the specific clinical characteristics of the patient. Extensive studies of the natural history and biomarkers of the disease will be required to achieve this goal.

Influence of lecithin cholesterol acyltransferase alteration during different pathophysiologic conditions: A 45 years bibliometrics analysis

Background: Lecithin cholesterol acyltransferase (LCAT) is an important enzyme responsible for free cholesterol (FC) esterification, which is critical for high density lipoprotein (HDL) maturation and the completion of the reverse cholesterol transport (RCT) process. Plasma LCAT activity and concentration showed various patterns under different physiological and pathological conditions. Research on LCAT has grown rapidly over the past 50 years, but there are no bibliometric studies summarizing this field as a whole. This study aimed to use the bibliometric analysis to demonstrate the trends in LCAT publications, thus offering a brief perspective with regard to future developments in this field. Methods: We used the Web of Science Core Collection to retrieve LCAT-related studies published from 1975 to 2020. The data were further analyzed in the number of studies, the journal which published the most LCAT-related studies, co-authorship network, co-country network, co-institute network, co-reference and the keywords burst by CiteSpace V 5.7. Results: 2584 publications contained 55,311 references were used to analyzed. The number of included articles fluctuated in each year. We found that Journal of lipid research published the most LCAT-related studies. Among all the authors who work on LCAT, they tend to collaborate with a relatively stable group of collaborators to generate several major authors clusters which Albers, J. published the most studies (n = 53). The United States of America contributed the greatest proportion (n = 1036) of LCAT-related studies. The LCAT-related studies have been focused on the vascular disease, lecithin-cholesterol acyltransferase reaction, phospholipid, cholesterol efflux, chronic kidney disease, milk fever, nephrotic syndrome, platelet-activating factor acetylhydrolase, reconstituted lpa-i, reverse cholesterol transport. Four main research frontiers in terms of burst strength for LCAT-related studies including "transgenic mice", "oxidative stress", "risk", and "cholesterol metabolism "need more attention. Conclusion: This is the first study that demonstrated the trends and future development in LCAT publications. Further studies should focus on the accurate metabolic process of LCAT dependent or independent of RCT using metabolic marker tracking techniques. It was also well worth to further studying the possibility that LCAT may qualify as a biomarker for risk prediction and clinical treatment.

Beneficial Effects of Citrus Flavanones Naringin and Naringenin and Their Food Sources on Lipid Metabolism: An Update on Bioavailability, Pharmacokinetics, and Mechanisms

Naringin and naringin's aglycone naringenin belong to a subclass of flavonoids called flavanones. While many studies of pure naringenin and naringin and their food sources have shown beneficial health effects, including improved lipid metabolism, in animals and humans, the mechanisms underlying the lipid-lowering effects have not been completely understood. In recent years, multiple studies using various in vitro and rodent models have revealed new mechanisms underlying the hypolipidemic effects of naringin and naringenin, including regulation of lipid digestion, reverse cholesterol transport, and LDL receptor expression. In addition, naringin and naringenin show diverse effects in populations with different health conditions, such as obesity and diabetes. Furthermore, a novel naringin and naringenin enriched food source citrus bergamia (bergamot) and other citrus fruits have recently been studied for lipid-lowering effects in animal models and human clinical trials. In this review, we provide an update on recent advances on naringin and naringenin and their enriched food sources on lipid metabolism and underlying mechanisms. Because absorption, distribution, metabolism, and excretion, particularly in the presence of food matrix, impact the bioavailability, which in turn affects the bioactivities of these flavonoids in vivo, we also summarize new findings from the pharmacokinetics studies and on interplays between naringin and naringenin and gut microbiota.

A systematic review of the natural history and biomarkers of primary Lecithin:Cholesterol Acyltransferase (LCAT) deficiency

Syndromes associated with LCAT deficiency, a rare autosomal recessive condition, include fish-eye disease (FED) and familial LCAT deficiency (FLD). FLD is more severe and characterized by early and progressive chronic kidney disease (CKD). No treatment is currently available for FLD, but novel therapeutics are under development. Furthermore, although biomarkers of LCAT deficiency have been identified, their suitability to monitor disease progression and therapeutic efficacy is unclear, as little data exist on the rate of progression of renal disease. Here, we systematically review observational studies of FLD, FED, and heterozygous subjects, which summarize available evidence on the natural history and biomarkers of LCAT deficiency, in order to guide the development of novel therapeutics. We identified 146 FLD and 53 FED patients from 219 publications, showing that both syndromes are characterized by early corneal opacity and markedly reduced HDL-C levels. Proteinuria/hematuria were the first signs of renal impairment in FLD, followed by rapid decline of renal function. Furthermore, LCAT activity toward endogenous substrates and the percentage of circulating esterified cholesterol (EC%) were the best discriminators between these two syndromes. In FLD, higher levels of total, non-HDL, and unesterified cholesterol were associated with severe CKD. We reveal a nonlinear association between LCAT activity and EC% levels, in which subnormal levels of LCAT activity were associated with normal EC%. This review provides the first step toward the identification of disease biomarkers to be used in clinical trials and suggests that restoring LCAT activity to subnormal levels may be sufficient to prevent renal disease progression.

The P274S Mutation of Lecithin-Cholesterol Acyltransferase (LCAT) and Its Clinical Manifestations in a Large Kindred

RATIONALE & OBJECTIVE

Lecithin-cholesterol acyltransferase (LCAT) catalyzes the maturation of high-density lipoprotein. Homozygosity for loss-of-function mutations causes familial LCAT deficiency (FLD), characterized by corneal opacities, anemia, and renal involvement. This study sought to characterize kidney biopsy findings and clinical outcomes in a family with FLD.

STUDY DESIGN

Prospective observational study.

SETTING & PARTICIPANTS

2 (related) index patients with clinically apparent FLD were initially identified. 110 of 122 family members who consented to genetic analysis were also studied.

PREDICTORS

Demographic and laboratory parameters (including lipid profiles and LCAT activity) and full sequence analysis of the LCAT gene. Kidney histologic examination was performed with samples from 6 participants.

OUTCOMES

Cardiovascular and renal events during a median follow-up of 12 years. Estimation of annual rate of decline in glomerular filtration rate.

ANALYTICAL APPROACH

Analysis of variance, linear regression analysis, and Fine-Gray competing-risk survival analysis.

RESULTS

9 homozygous, 57 heterozygous, and 44 unaffected family members were identified. In all affected individuals, full sequence analysis of the LCAT gene revealed a mutation (c.820C>T) predicted to cause a proline to serine substitution at amino acid 274 (P274S). Homozygosity caused a complete loss of LCAT activity. Kidney biopsy findings demonstrated lipid deposition causing glomerular basement membrane thickening, mesangial expansion, and "foam-cell" infiltration of kidney tissue. Tubular atrophy, glomerular sclerosis, and complement fixation were associated with worse kidney outcomes. Estimated glomerular filtration rate deteriorated among homozygous family members at an average annual rate of 3.56 mL/min/1.73 m². The incidence of cardiovascular and renal complications was higher among homozygous family members compared with heterozygous and unaffected members. Mild thrombocytopenia was a common finding among homozygous participants.

LIMITATIONS

The presence of cardiovascular disease was mainly based on medical history.

CONCLUSIONS

The P274S LCAT mutation was found to cause FLD with renal involvement. Tubular atrophy, glomerular sclerosis, and complement fixation were associated with a worse renal prognosis.

Cluster expansion of apolipoprotein D (ApoD) genes in teleost fishes

BACKGROUND

Gene and genome duplication play important roles in the evolution of gene function. Compared to individual duplicated genes, gene clusters attract particular attention considering their frequent associations with innovation and adaptation. Here, we report for the first time the expansion of the apolipoprotein D (ApoD) ligand-transporter genes in a cluster manner specific to teleost fishes.

RESULTS

Based on comparative genomic and transcriptomic analyses, protein 3D structure comparison, positive selection detection and breakpoints detection, the single ApoD gene in the ancestor expanded into two clusters following a dynamic evolutionary pattern in teleost fishes. Orthologous genes show conserved expression patterns, whereas lineage-specific duplicated genes show tissue-specific expression patterns and even evolve new gene expression profiles. Positive selection occurred in branches before and after gene duplication, especially for lineage-specific duplicated genes. Cluster analyses based on protein 3D structure comparisons, especially comparisons of the four loops at the opening side, show gene duplication-segregating patterns. Duplicated ApoD genes are predicted to be associated with forkhead transcription factors and MAPK genes. ApoD clusters are located next to the breakpoints of genome rearrangements.

CONCLUSIONS

Here, we report the expansion of ApoD genes specific to teleost fishes in a cluster manner for the first time. Neofunctionalization and subfunctionalization were observed at both the protein and expression levels after duplication. Evidence from different aspects-i.e., abnormal expression-induced disease in humans, fish-specific expansion, predicted associations with forkhead transcription factors and MAPK genes, specific expression patterns in tissues related to sexual selection and adaptation, duplicated genes under positive selection and their location next to the breakpoints of genome rearrangements-suggests the potentially advantageous roles of ApoD genes in teleost fishes. The cluster expansion of ApoD genes specific to teleost fishes provides thus an ideal evo-devo model for studying gene duplication, cluster maintenance and new gene function emergence.

A thumbwheel mechanism for APOA1 activation of LCAT activity in HDL

APOA1 is the most abundant protein in HDL. It modulates interactions that affect HDL's cardioprotective functions, in part via its activation of the enzyme, LCAT. On nascent discoidal HDL, APOA1 comprises 10 α-helical repeats arranged in an anti-parallel stacked-ring structure that encapsulates a lipid bilayer. Previous chemical cross-linking studies suggested that these APOA1 rings can adopt at least two different orientations, or registries, with respect to each other; however, the functional impact of these structural changes is unknown. Here, we placed cysteine residues at locations predicted to form disulfide bonds in each orientation and then measured APOA1's ability to adopt the two registries during HDL particle formation. We found that most APOA1 oriented with the fifth helix of one molecule across from fifth helix of the other (5/5 helical registry), but a fraction adopted a 5/2 registry. Engineered HDLs that were locked in 5/5 or 5/2 registries by disulfide bonds equally promoted cholesterol efflux from macrophages, indicating functional particles. However, unlike the 5/5 registry or the WT, the 5/2 registry impaired LCAT cholesteryl esterification activity (P < 0.001), despite LCAT binding equally to all particles. Chemical cross-linking studies suggest that full LCAT activity requires a hybrid epitope composed of helices 5-7 on one APOA1 molecule and helices 3-4 on the other. Thus, APOA1 may use a reciprocating thumbwheel-like mechanism to activate HDL-remodeling proteins.

Role of apolipoproteins, ABCA1 and LCAT in the biogenesis of normal and aberrant high density lipoproteins

In this review, we focus on the pathway of biogenesis of HDL, the essential role of apoA-I, ATP binding cassette transporter A1 (ABCA1), and lecithin: cholesterol acyltransferase (LCAT) in the formation of plasma HDL; the generation of aberrant forms of HDL containing mutant apoA-I forms and the role of apoA-IV and apoE in the formation of distinct HDL subpopulations. The biogenesis of HDL requires functional interactions of the ABCA1 with apoA-I (and to a lesser extent with apoE and apoA-IV) and subsequent interactions of the nascent HDL species thus formed with LCAT. Mutations in apoA-I, ABCA1 and LCAT either prevent or impair the formation of HDL and may also affect the functionality of the HDL species formed. Emphasis is placed on three categories of apoA-I mutations. The first category describes a unique bio-engineered apoA-I mutation that disrupts interactions between apoA-I and ABCA1 and generates aberrant preβ HDL subpopulations that cannot be converted efficiently to α subpopulations by LCAT. The second category describes natural and bio-engineered apoA-I mutations that generate preβ and small size α4 HDL subpopulations, and are associated with low plasma HDL levels. These phenotypes can be corrected by excess LCAT. The third category describes bio-engineered apoA-I mutations that induce hypertriglyceridemia that can be corrected by excess lipoprotein lipase and also have defective maturation of HDL. The HDL phenotypes described here may serve in the future for diagnosis, prognoses and potential treatment of abnormalities that affect the biogenesis and functionality of HDL.

A Systematic Investigation of Structure/Function Requirements for the Apolipoprotein A-I/Lecithin Cholesterol Acyltransferase Interaction Loop of High-density Lipoprotein

The interaction of lecithin-cholesterol acyltransferase (LCAT) with apolipoprotein A-I (apoA-I) plays a critical role in high-density lipoprotein (HDL) maturation. We previously identified a highly solvent-exposed apoA-I loop domain (Leu(159)-Leu(170)) in nascent HDL, the so-called "solar flare" (SF) region, and proposed that it serves as an LCAT docking site (Wu, Z., Wagner, M. A., Zheng, L., Parks, J. S., Shy, J. M., 3rd, Smith, J. D., Gogonea, V., and Hazen, S. L. (2007) Nat. Struct. Mol. Biol. 14, 861-868). The stability and role of the SF domain of apoA-I in supporting HDL binding and activation of LCAT are debated. Here we show by site-directed mutagenesis that multiple residues within the SF region (Pro(165), Tyr(166), Ser(167), and Asp(168)) of apoA-I are critical for both LCAT binding to HDL and LCAT catalytic efficiency. The critical role for possible hydrogen bond interaction at apoA-I Tyr(166) was further supported using reconstituted HDL generated from apoA-I mutants (Tyr(166) → Glu or Asn), which showed preservation in both LCAT binding affinity and catalytic efficiency. Moreover, the in vivo functional significance of NO2-Tyr(166)-apoA-I, a specific post-translational modification on apoA-I that is abundant within human atherosclerotic plaque, was further investigated by using the recombinant protein generated from E. coli containing a mutated orthogonal tRNA synthetase/tRNACUA pair enabling site-specific insertion of the unnatural amino acid into apoA-I. NO2-Tyr(166)-apoA-I, after subcutaneous injection into hLCAT(Tg/Tg), apoA-I(-/-) mice, showed impaired LCAT activation in vivo, with significant reduction in HDL cholesteryl ester formation. The present results thus identify multiple structural features within the solvent-exposed SF region of apoA-I of nascent HDL essential for optimal LCAT binding and catalytic efficiency.

Structural Insights into High Density Lipoprotein: Old Models and New Facts

The physiological link between circulating high density lipoprotein (HDL) levels and cardiovascular disease is well-documented, albeit its intricacies are not well-understood. An improved appreciation of HDL function and overall role in vascular health and disease requires at its foundation a better understanding of the lipoprotein's molecular structure, its formation, and its process of maturation through interactions with various plasma enzymes and cell receptors that intervene along the pathway of reverse cholesterol transport. This review focuses on summarizing recent developments in the field of lipid free apoA-I and HDL structure, with emphasis on new insights revealed by newly published nascent and spherical HDL models constructed by combining low resolution structures obtained from small angle neutron scattering (SANS) with contrast variation and geometrical constraints derived from hydrogen-deuterium exchange (HDX), crosslinking mass spectrometry, electron microscopy, Förster resonance energy transfer, and electron spin resonance. Recently published low resolution structures of nascent and spherical HDL obtained from SANS with contrast variation and isotopic labeling of apolipoprotein A-I (apoA-I) will be critically reviewed and discussed in terms of how they accommodate existing biophysical structural data from alternative approaches. The new low resolution structures revealed and also provided some answers to long standing questions concerning lipid organization and particle maturation of lipoproteins. The review will discuss the merits of newly proposed SANS based all atom models for nascent and spherical HDL, and compare them with accepted models. Finally, naturally occurring and bioengineered mutations in apoA-I, and their impact on HDL phenotype, are reviewed and discuss together with new therapeutics employed for restoring HDL function.